The invention relates to the field of PMD emulators and in particular to a combinatorial approach to build polarization scramblers in a many-segment PMD emulator.
As the telecommunication industry continues to deploy higher bandwidth fiber optic systems, Polarization Mode Dispersion (PMD) becomes one of the major obstacles in 10 Gbit/s and higher transmission systems. The evaluation of PMD mitigation techniques requires a method of emulating the PMD of the installed fiber links, especially since “legacy” fiber with high PMD installed in the 1980's is no longer commercially available. PMD emulation has thus become an active area of research since the last few years. Much work has focused on emulating PMD using a concatenation of many birefringent segments with random coupling at each junction.
One important requirement of these emulators is that the number of segments concatenated has to be large. This is to give the key properties of a good PMD emulator which include (i) a Maxwellian distributed DGD, (ii) accurate higher-order PMD statistics and (iii) a frequency autocorrelation function that tends toward zero after a frequency range that spans a few times the bandwidth of the principle state of polarization (PSP). On the other hand, practical considerations such as the size of the emulator, the cost of building it, the ease of implementation, the complexity involved in the controls and the insertion loss often limit the number of segments to typically about 15.
A polarization scrambler placed between segments is the one of the major components that determines its cost, size and complexity. Since the number of polarization scramblers required is linearly proportional to the number of segments used, these factors scale up linearly with the number of segments. It has been demonstrated that a dynamic PMD emulator using a total of 36 squeezers (3 squeezers in each of the 12 polarization controllers) while another experiment demonstrated using 64 motors to control the 64 fiber twisters. The cost to build and the complexity to control such many squeezers are substantial.